The present invention is related to an improved method of forming a solid electrolytic capacitor and an improved capacitor formed thereby. More specifically, the present invention is related to materials and methods for improving corner and edge coverage of solid electrolytic capacitors. The invention also discloses methods for manufacturing the same.
The construction and manufacture of solid electrolyte capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the valve metal is electrolytically formed to cover all surfaces of the anode and serves as the dielectric of the capacitor. The solid cathode electrolyte is typically chosen from a very limited class of materials, to include manganese dioxide or electrically conductive organic materials such as polyaniline, polypyrrole, polythiophene and their derivatives. Solid electrolytic capacitors with intrinsically conductive polymers as the cathode material have been widely used in the electronics industry due to their advantageously low equivalent series resistance (ESR) and “non-burning/non-ignition” failure mode. In the case of conductive polymer cathodes the conductive polymer is typically applied by either chemical oxidation polymerization, electrochemical oxidation polymerization or spray techniques with other less desirable techniques being reported.
The backbone of a conductive polymer comprises a conjugated bonding structure. The polymer can exist in two general states, an undoped, non-conductive state, and a doped, conductive state. In the doped state, the polymer is conductive but of poor processibility due to a high degree of conjugation along the polymer chain. In its undoped form, the same polymer loses its conductivity but can be processed more easily because it is more soluble. When doped, the polymer incorporates anionic moieties as constituents on its positively charged backbone. In order to achieve high conductivity, the conductive polymers used in the capacitor must be in doped form after the completion of the process although, during the process the polymer can be undoped/doped to achieve certain process advantages.
Various types of conductive polymers including polypyrrole, polyaniline, and polythiophene are applied to the capacitors. The major drawback of conductive polymer capacitors, regardless of the types of conductive polymers employed, is their relatively low working voltage compared to their MnO2 counterparts. The polymer capacitors have reliability issues, to varying degrees, when the voltage rating exceeds 25V. This is believed to be caused by the relatively poor dielectric-polymer interface, which has poor “self-healing” capability. The ability to withstand high voltage can be best characterized by the breakdown voltage (BDV) of the capacitors. Higher BDV corresponds with better reliability. For reasons which were previously unknown, the break down voltage of capacitors comprising conductive polymers has been limited to about 55V thereby leading to a capacitor which can only be rated for use at about 25V. This limitation has thwarted efforts to use conductive polymers more extensively.
U.S. Pat. No. 7,563,290, which is incorporated herein by reference, describes the polymer slurry process wherein a conducting polymer dispersion is deposited such as by dipping. The resulting capacitors show excellent high voltage performances, reduced DC leakage (DCL) and improved long term reliability.
It is highly desirable that the capacitor devices are of high reliability and that they can withstand stressful environments. Therefore, the integrity of the anodes and the robustness of conductive polymer cathodes are essential for high quality capacitor products. However, it is a challenge to form a conductive polymer coating on the anodes that is defect-free, and which is capable of withstanding thermal mechanical stress during anode resin encapsulation and surface-mounting. The improper application of polymer slurry often leads to the formation of a poor polymer coating.
In a manufacturing process to produce conductive polymer based valve metal capacitors the valve metal powder, such as tantalum, is mechanically pressed into anodes that are subsequently sintered to form porous bodies. The anodes are anodized to a pre-determined voltage in a liquid electrolyte to form a dielectric layer onto which a cathode layer of conductive polymer is subsequently formed via an in situ polymerization process comprising multi-cycle oxidizer/monomer coatings and polymerization reactions. The anodes are then coated with graphite and Ag followed by assembling and molding into a finished device.
A particular concern is the formation of adequate polymer coatings on edges and corners. In order to achieve good quality polymer coating on anodes, especially on the corners and edges, many types of chemical compounds are used for either forming a pre-coating on the anode or which are added to the polymer slurry. U.S. Pat. No. 7,658,986, which is incorporated herein by reference, describes the difficulty in coating the edges and corners of the anode with polymer slurry. These materials tend to pull away from the corners and edges due to surface energy effects. The resulting thin coverage at corners and edges leads to poor reliability of the device.
One approach to mitigating poor coverage of the anode corners and edges has been to alter the design of the anode as disclosed in U.S. Pat. No. 7,658,986, D616,388, D599,309, and D586,767 each of which is incorporated herein by reference. While changes in the anode design are beneficial in some regards, the effect of poor coverage is still present even with anode designs facilitating corner and edge coverage by polymer slurry as the primary cathode layer.
Another approach for improving coverage of the corners and edges is provided in U.S. Pat. No. 8,882,856, which is incorporated herein by reference, which describes a group of chemical compounds called crosslinkers, which are mostly multi-cationic salts or amines. This patent teaches the application of a solution of the crosslinker between adjacent layers of conducting polymer dispersion coating to achieve good polymer coverage on corners and edges of the anodes. The effectiveness of the crosslinker is attributed to the cross-linking ability of multi-cationic salts or amines to the slurry/dispersion particles. While crosslinkers are advantageous for improving the coating coverage on corners and edges of the anodes, the addition of these crosslinkers, which are mostly ionic in nature, has the unintended consequences of degrading the humidity performance of a finished product.
Crosslinkers, by definition, are multi-functional and link one polymer chain, or portion of a chain, to another thus tending to be part of the polymer system. While crosslinkers are advantageous in many applications, it is undesirable to have an ionic crosslinker react to the polymer chain and be part of the polymer chain. Ionic materials, especially low molecular weight ionic compounds or mobile ionic compounds, can diffuse though various cathode layers, especially under humid conditions, and can cause higher leakage current.
U.S. Pat. No. 8,771,381, which is incorporated herein by reference, teaches the use of nonionic polyol as a precoat to conducting polymer dispersion to eliminate the use of ionic compounds as well as improving the coverage
U.S. Pat. No. 9,030,807, which is incorporated herein by reference, teaches a method for avoiding crosslinking of ionic compounds as well as for improving the coverage by the use of a class of latent acid curing catalysts such as amine blocked sulphonic acids and certain class of nanoparticles between the layers of conducting polymer dispersions.
The prior art mentioned above uses an insulating compound between conducting polymer dispersions to improve the coverage. Having an insulating material between conducting polymer can affect Equivalent Series Resistance (ESR) and other electrical properties. So a need exists for materials and methods which improves corner and edge coverage while not impacting the electrical properties adversely. An advancement in the use of crosslinkers for formation of a cathode layer on solid electrolytic capacitors is provided herein.